1. Field of the Invention
The present invention relates generally to nuclear reactors, and more particularly, to nuclear reactors having fuel assemblies that employ grids.
2. Description of the Related Art
In most water cooled nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. In pressurized water nuclear reactors, these fuel assemblies typically include a plurality of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated thimble tubes. The thimble tubes typically receive control rods or instrumentation therein. Top and bottom nozzles are on opposite ends of the fuel assembly and are secured to the ends of the thimble tubes that extend slightly above and below the ends of the fuel rods.
During manufacture, subsequent installation and repair of components of the nuclear reactor coolant circulation system, a diligent effort is made to help assure the removal of all debris from the reactor vessel and its associated systems, which circulate coolant throughout the primary reactor coolant loop under various operating conditions. Although elaborate procedures are carried out to help assure debris removal, experience shows that in spite of the safeguards used to effect such removal, some chips and metal particles still remain hidden in the system. Most of the debris consists of metal turnings, which were probably left in the primary system after steam generator repair or replacement.
Fuel assembly damage due to debris trapped at the lower most grid has been noted in several reactors. Debris enters through the fuel assembly bottom nozzle flow holes from the coolant flow openings in the lower core support plate when the plant is started up. The debris tends to be engaged in the lower most support grid of the fuel assembly within the spaces between the fuel rod support cell walls of the grid and the lower end portions of the fuel rod tubes. The damage consists of fuel rod tube perforations caused by fretting of the debris in contact with the exterior of the cladding tubes which sealably enclose the fissile material in the fuel rod. Debris also becomes entangled in the lower nozzle plate holes. The flowing coolant causes the debris to gyrate, which tends to cut through the cladding of the fuel rods.
Due to the potential for debris to damage components of the fuel assembly, it is known to additionally provide a protective grid that is disposed between the bottom support grid and the lower nozzle. The protective grid functions as a filter to protect the fuel rods from debris. It is also known to securely dispose the protective grid against the lower nozzle and to space the protective grid from the lower nozzle in order to avoid undesirable contact between the lower nozzle and irregularities on the edges of the assembled straps of the grid.
All of the grids of the fuel assemblies of the nuclear reactor, including the protective grids, are typically made up of a plurality of straps that are arranged in a lattice pattern and are fastened to one another to define a plurality of cells. The cells include control rod guide thimble cells and fuel rod cells. The top support grid, bottom support grid and middle support grids are mechanically or otherwise fastened to the thimble tubes which are disposed in the thimble cells. The fuel rods typically are held within the fuel rod cells of the grids, including the protective grid, by a plurality of springs and/or dimples within each fuel rod cell that are formed in the straps of the grids.
The protective grid typically has spacers that are welded or otherwise attached to the underside of the grid and are securely captured between the protective grid and the fuel assembly bottom nozzle.
Cracking has recently been experienced in the protective grid features which has led to fuel rod failures. These failures have been determined to be caused by intergranular cracking, consistent with stress corrosion cracking. One of the sources of stress is the differential thermal expansion between the bottom nozzle and the protective grid, since the bottom nozzle is made of stainless steel and the protective grid is made of alloy-718. These parts are rigidly connected via a thimble screw which is screwed through the underside of the bottom nozzle and into the end plug on the control rod guide thimble. The stainless steel expands more during heat up than does the alloy-718, thus causing stresses in the protective grid, particularly near the attachment point.
Accordingly, a new means of attaching the protective grid is desired that will avoid the build-up of stresses during heat-up of the reactor.